Oscillating frequency detector



Aug. 19, 1947. E. LABIN 2,426,030

OSCILLATING FREQUENCY DETECTOR Filed Dec. 2, 1943 WWn n n n,,, [W n nnnn;

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ATTORNEY Patented Aug. 19, 1947 UNITED STATES PATENT OFFICE OSCELATIN G FREQUENCY DETECTOR Edouard Labin, Buenos Aires, Argentina, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application December 2, 1943, Serial No. 512,617

4 Claims.

The present invention relates to frequency detection and more particularly to an oscillating frequency detector, especially useful for frequency stabilization purposes.

As is Well known, in common frequency modulation systems, where the only function of the frequency detector consists of detecting the frequency variations of the received signal, it is quite suflicient to transform these frequency deviations into a low-frequency potential varying in amplitude around som mean value which may be positive or negative with respect to the ground potential of th receiving system.

However, in frequency modulation systems comprising frequency stabilizing links, the frequency detector is usually designed to operate as an audio frequency detector and to provide at the same time the control potential for the frequency stabilizing link. Therefore, the output characteristics of these frequency detectors should comply with certain specifications if a satisfactory performance is to be obtained.

As is well known in the art, frequency detectors of the latter type should not only provide a zero output potential with respect to the ground for the mean frequency of the frequency modulation system, but should also have a zero output potential in the absence of the signal, so that the frequency of the adjustable controlled local oscillator may slide back or return to its central operating frequency when the signal fades out or during interruptions in the transmission.

One type of frequency detector which has been previously used in frequency modulation systems utilizes the frequency response characteristics of selective circuits, but although such tuned frequency detectors or discriminators fulfill the aforementioned requirements, the stability of their zero adjustment depends upon the quality of the tuned circuits used, and consequently such frequency detectors are not particularly suitable for frequency stabilizing systems,

Furthermore, in the tuned frequency discriminators the amplitude of the output potential depends not only upon the frequency deviation of the received signal from its mean value, but also on the amplitude of said signal, so that specially designed limiter stages have to be used in order to overcome this serious drawback.

Therefore, it is one of the objects of the present invention to provide an improved frequency detector which is characterized by the advantage of generating a zero output potential in the ab sence of the signal and for a predetermined frequency of the oscillation to be detected, the sta- 2 bility of the discriminator characteristic curve thereof remaining uninfiuenced by structural changes of the circuit elements.

It is a further object of the present invention to provide a frequency detector of simple circuit arrangement comprising a minimum of circuit elements.

Yet a further object of the present invention is to provide a frequency detector which will not only generate an output potential proportional to the frequency of the applied oscillation over a Wide range of frequency excursions but the output potential of which is also zero for frequency excursions beyond the operating range of the detector.

According to the invention, the frequency detector comprises an oscillator designed to generate electrical impulses of constant area and amplitude at time intervals which, in the presence of the signal oscillation, are controlled by the half waves of like sign of said oscillation, while in the absence of such oscillation, the time interval between said impulses corresponds to some predetermined frequency of said signal oscillation, Hence, the generated impulses will not only be locked in by the signal frequency applied to the input terminals of the oscillating frequency detector, following exactly the frequency variations thereof, but will automatically return to the frequency of free oscillation when the .signal disappears.

Since the area of the generated impulses is constant and independent of the frequency, it will be evident to those skilled in the art that a low frequency component proportional to the frequency of the signal oscillation may be derived from the output of said oscillating impulse generator by means of a convenient filter circuit or other adequate device. By inserting a compensating voltage in the output circuit of the oscillating frequency detector it is therefore possible to provide a zero output potential not only for a predetermined frequency of the signal oscillation but also in the absence thereof. The compensating voltage is applied in a manner similar to that described in U; S. patent applications Serial No. 464,380, filed November 3, 1942, now U. S. Patent 2,406,309, issued August 20, 1946, and Serial No. 477,990, filed March 4, 1943.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, in which:

Fig. 1 is a schematic diagram of a compensated 3 oscillating frequency detector according to the present invention,

Figs. 2A, 2B and 2C are graphs illustrating the generated impulses at time intervals corresponding to the natural frequency and to a controlled or locked-in higher and lower frequency, respectively,

Fig. 3 is a graph illustrating the direct output potential derived from the oscillating impulse generator shown in block form in Fig. 1,

Fig. 4 is a graph illustrating the output potential developed between the output terminal of the frequency detector shown schematically in Fig. 1.

The same reference characters indicate like or corresponding elements throughout the drawings.

Referring now to the drawing, it can be seen that the oscillating frequency detector, according to the present invention, comprises an impulse generator lil provided with input terminals II and [2 to which an electrical oscillation f of variable frequency is applied, the output of said impulse generator l being coupled to a filter l3 provided with output terminals I4 and [5.

As already explained hereinbefore, impulse generator It! is designed to generate impulse S of constant area and amplitude at time intervals T (Fig. 2A) so that the fundamental frequency of the freely generated impulses corresponds to the mean frequency fm of said oscillation I. However, when oscillation is applied to the input terminals H and I2 of the impulse generator I9, the time intervals T between said impulses S will be controlled by the half waves of like sign of said oscillation 1 without changing the area or amplitude of the same. Consequently, smaller time intervals Th will correspond to a higher frequency of oscillation f, while larger time intervals T1 are obtained when the frequency of oscillation ,f is lower than fm (Figs. 2B and 2C) By designing filter 13 as a low pass filter, a potential V is produced which will deviate from its mean value proportionally to the variations of instantaneous time intervals between the generated impulses S and consequently proportionally to the frequency oscillation f. The mean potential Vu corresponding to the mean frequency fm will be determined by the area of the impulses S and is proportional to the voltage of the impulse generator direct current supply, and the frequency discrimination characteristic obtained will be as shown in Fig. 3.

The oscillating frequency detector of Fig. 1 also embodies a compensating direct current potential Vc derived by means of potentiometer 29 from the main direct current supply 28 of impulse generator i0. The potentiometer relation is so adjusted that for the mean frequency fm of oscillation the output potential V'=V0Vc will be zero independently of fluctuations in the voltage of the main direct current supply 28, while for frequencies higher or lower than he a positive or negative output potential V Will be obtained.

With oscillation 1 applied to input terminals H and [2 of the oscillating frequency detector, the generated impulses will exactly follow the half wave of like sign of said oscillation, so that the output potential V developed across the output terminals l4 and I5, will be proportional to the instantaneous frequency of oscillation f. If, however, said oscillation f disappears due to fading or other circumstances, the generated impulses S automatically return a frequency corresponding to the free oscillation of the generator Ill, and in view of the fact that this frequency is substantially equal to the mean frequency fm, the output potential V is zero even in the absence of oscillation 1.

Since the area and amplitude of the generated impulses S do not depend upon the amplitude of the oscillation f, the output potential developed by the oscillating frequency detector is independent of the amplitude variations of the oscillation to be detected, rendering the use of any amplitude limiter unnecessary. On the other hand, the operating range of the oscillating frequency detector is chiefly determined by the range over which a synchronization of the generated impulses S and oscillation f can be maintained. If, during excessive frequency swings, oscillation 1 fails to lock-in or synchronize the generated impulses S, the output potential of the oscillating frequency detector slides back to zero, since the impulses S return to the frequency corresponding to the free oscillation of impulse generator l0. Consequently, the output voltage is zero beyond the operating range of the frequency detector, and the frequency discriminator characteristic so obtained, and shown in Fig. 4, can be regarded as the ideal output characteristics for a frequency discriminator designed for frequency stabilization purposes.

Fig. 1 represents an oscillating frequency detector of the compensated type in which the impulse generator designated by the general reference number IO, is formed by a thermionic tube 23 connected as a blocked or damped oscillator, the circuit elements being so arranged that the oscillation generated consists of a series of impulses, the time. intervals between said impulses being mainly determined by grid-leak resistance Tg, condenser Cg and resistance r connected across the primary winding 24 of transformer T, the secondary winding 25 of which is connected to the anode of said tube 23.

Inputterminals ll and I2 are connected to the ends of a resistance 26 inserted in series with said primary winding 24, so that the half waves of like sign of the oscillation f, applied to input terminals II and I2, control the start of each of the generated impulses, without changing the area or amplitude of the same.

The plate circuit of tube 23 also comprises a load resistance 21 inserted between the secondary winding 25 of transformer T and the positive pole of a direct current supply 28 which is shunted by a potentiometer 29, the output circuit of the blocked oscillator being constituted by said load resistance 21 and the adjacent portion of potentiometer 29.

This output circuit is coupled to a filter l3 for deriving from the generated impulses an output potential proportionally related to the fundamental frequency of said impulses and hence to the frequency of oscillation controlling the time intervals between said impulses. In View of the particular arrangement of the plate circuit of tube 23 wherein a potential is derived from the potentiometer 29 and injected in series with the potential across resistor 21, the output potential developed across output terminals I4 and [5 will be zero when oscillation f passes through the mean frequency fm, and consequently the frequency discriminator curve of this compensated frequency detector is similar to that shown in Fig. 4.

It will be evident for those skilled in the art that a large variety of oscillating devices may be used as impulse generators for the oscillating frequency detector according to the present invention, which therefore is not to be construed as limited to the particular circuits herein disclosed, since these are merely illustrative, but the scope of the invention is indicated in the appended claims.

I claim:

1. A method of detecting the frequency of an electrical oscillation varying around a given mean frequency, which comprises the steps of locally generating electrical impulses of like sign at conrtrollable time intervals in a circuit comprising a source of direct current, said impulses having an area proportionally related to the direct-current voltage of said source and a duration shorter than the periods of the highest frequency to be detected, the time intervals between the impulses corresponding to the natural frequency of the impulse oscillation being equal to the periods of said mean frequency, controlling said time intervals in synchronism with the half Waves of like sign of said oscillation to obtain an impulse wave the fundamental frequency of which follows the frequency variations of said oscillation, integrating said synchronized impulses to produce an output voltage proportional to said fundamental frequency and to said direct-current voltage, and simultaneously adding a fraction of said directcurrent voltage in opposition to said output voltage to produce a net magnitude of said output voltage which is zero at values of said frequency equal to said mean frequency, for said mean frequency, in the absence of said oscillation and beyond the synchronizability range of said impulses and which is substantially independent of variations in said direct current source.

2. A compensated oscillating frequency discriminator for detecting the frequency of an electrical oscillation varying around a given mean value, which comprises a source of direct current, means for generating electrical impulses of like sign at controllable time intervals, said generated means having an input circuit and an output circuit, said controllable time intervals being substantially equal to the periods of said mean frequency and having an area substantially proportional to the potential of said source and a duration shorter than the periods of the highest frequency of the oscillation to be detected, the input circuit of said impulse generating means comprising means for controlling said time intervals in accordance with the half waves of like sign of said oscillation, the output of said impulse generating means being coupled to means for integrating said impulses to obtain an output voltage the variations of which around said mean value are proportionally related to the frequency variations of said oscillation around said mean frequency and to said potential of said source, means to connect a portion of the direct current potential obtained from said source in opposition to said output voltage, to produce a net magnitude of said output voltage which is compensated to Zero indedendently of variations in said direct current source and in the absence of said oscillation, for the mean value of the frequency of said oscillation, and for values of said frequency beyond the range of synchronizability of said impulse generating means.

3. A method of detecting frequency variations of an incoming electrical oscillation about a given frequency value, comprising the steps of generating a synchronizable wave of electrical impulses 5 of like sign at an uncontrolled frequency rate equal to the given frequency value, said impulses having an area proportional to a given direct ourrent voltage and a duration shorter than the period of the highest frequency to be detected, controlling the frequency of said impulses in synchronism with the incoming oscillation, deriving from said impulses a potential proportional to the frequency of the incoming oscillation, and. simultaneously adding a direct current potential proportional to the area of said impulses in opposition to said derived potential to produce an output voltage the amplitude of which varies proportionally to the frequency variations of the incoming oscillation and which is substantially zero at frequency values of the incoming oscillation equal to the predetermined value and beyond the synchronizing range of the locally generated wave.

4. A compensated oscillating frequency detector comprising an impulse generator, a direct current supply and a low pass filter, said impulse generator comprising a thermionic tube having input and output electrodes, a transformer having a primary winding and a secondary winding coupled to said tube to produce a blocking oscillator for generating electrical impulses of like sign at controllable time intervals,-said impulses having an area proportional to the potential of said direct current supply and being shorter in duration than the periods of the highest frequency of the oscillation to be detected, the time interval of said impulses corresponding to the free oscillation of said generator being equal to the periods of the mean frequency of said oscillation, a resistor connected in series with said primary Winding, input terminals connected across said resistor, a load resistor coupled to the output electrode of said tube through said secondary winding and connected to the positive pole of said direct cur- 45 rent supply and a voltage divider having an adjustable tap shunted across said direct current supply, said low pass filter being coupled between the junction point of said load resistor and said secondary winding for integrating the impulses 50 developed across said load resistor to obtain an output voltage which is proportional to the frequency of the oscillation applied to said input terminals, which is zero in the absence of said oscillation and for frequency variations beyond 55 the range of synchronizability of said blocking oscillator, and is substantially independent of variations in said direct current supply.

EDOUARD LABIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,284,444 Peterson May 26, 1942 2,323,596 Hansell July 6, 1943 2,296,056 Roberts Sept. 15, 1942 

